1. Field of the Invention
The present invention relates to a circuit for calibrating a signal, that is, for providing, based on an input signal, a signal included in a predetermined range.
2. Description of the Related Art
There are various applications for calibration circuits. For example, calibration circuits are used in optical disk read systems. In such systems, a calibration circuit is arranged between a photodetector and an analog-to-digital converter to adapt the signal from the photodetector to the analog-to-digital converter input range. The integrated circuit “L6300” of STMicroelectronics is an example of a calibration circuit used in such a system.
FIG. 1 illustrates the shape, along time, of a voltage V at the input of a calibration circuit. Voltage V, shown as a sinusoid, oscillates between a minimum value Vmin and a maximum value Vmax. In the mentioned application, voltage V coming from a photodetector oscillates above a reference level Vmin approximately equal to 2.5 volts. Peak-to-peak amplitude Vmax–Vmin of voltage V can vary within a large range from 25 to 500 millivolts, according to whether the photodetector receives all or part of the light emitted by the laser.
FIG. 2 illustrates the shape, along time, of voltage VOUT at the output of the calibration circuit. Voltage VOUT has the same frequency as voltage V, but its amplitude is constant and the signal oscillates between a minimum value Vbot and a maximum value Vtop corresponding to desired limiting values. Typical values of Vbot and Vtop, in the application mentioned hereabove, are respectively around 125 and 875 millivolts.
Generally, the forming of a calibration circuit must take into account error or offset voltages likely to affect the limiting values of output voltage VOUT. For example, in the mentioned application, an error voltage which can reach ±70 millivolts systematically affects the reference level Vmin. Further, each element of the calibration circuit introduces an error voltage specific to it. This effect is particularly substantial if the circuit includes MOS transistors, since these transistors generate greater error voltages than bipolar transistors.
Because of this, many calibration circuits, like circuit “L6300” mentioned hereabove, are formed by means of bipolar transistors. Further, since the signal processing circuits that follow the calibration circuit are generally formed by means of CMOS transistors, the calibration circuit can hardly be formed together with the circuits that follow it on a same silicon wafer to form an integrated circuit. This results in high manufacturing, testing, and interconnection costs.
Further, known calibration circuits are powered by relatively high supply voltages, greater than those supplying CMOS circuits.